As reflection-type image display elements for use in projectors, there are known digital micromirror devices. A digital micromirror device has a picture display surface composed of a plurality of micromirrors, which are minute mirrors; it controls the tilt of each mirror surface on the image display surface, thereby modulates the intensity of illumination light, and thereby forms an image. That is, in a digital micromirror device, the turning on and off of each pixel is achieved, for example, by the rotation of a mirror surface through ±12° about a rotation axis disposed at 45° to each side of the image display surface. As for the movement of the micromirrors, Nonpatent Literature 1 listed below proposes a digital micromirror device of a new operation type.
With a reflection-type image display element such as a digital micromirror device, there occurs unnecessary light called OFF light, which is not used to form a projection image. The OFF light causes degradation of contrast in the projection image. More specifically, illumination light travels through a gap formed by the micromirrors tilted in an OFF state, and sneaks into a mirror back surface side, where the illumination light is irregularly reflected to become stray light, which causes contrast degradation. Nonpatent Literature 2 listed below describes covering the micromirror back surface side with a black oxide film in order to reduce the occurrence of such stray light. Furthermore, as methods for improving the contrast, Patent Literature 1 proposes shifting an angle of illumination light and using a noncircular aperture stop, and Patent Literature 2 proposes using a stop having an aperture elongated in a pixel tilt axis direction.